The invention relates to the art of screen printing in general, and more particularly to improvements in apparatus for making screens or stencils for use in screen printing machines. Still more particularly, the invention relates to improvements in apparatus for stretching woven or other suitable fabrics over the frames of screens or stencils of the type suitable for use in screen printing machines.
Screen printing is resorted to in a number of different fields, particularly in fields where the application of a design or pattern upon a substrate must be carried out with a very high degree of precision. Typical examples of such substrates are printed circuit boards to which a design must be applied with tolerances of not more than 0.05 mm. The mesh of the fabric must be maintained within tolerances in the range of one or more thousandths of one millimeter.
The making of a stencil or screen which is suitable for use in connection with the making of printed circuit boards and in other fields wherein a high degree of precision is equally or even more important necessitates highly uniform and predictable stretching of a fabric (such fabrics are often woven from silk, polyester, nylon or stainless steel) upon a normally rectangular frame which can be made of wood, aluminum or other suitable material. Even minor distortions which are attributable to non-uniform stretching of the fabric will render the thus obtained stencils useless for their intended purpose because the tolerances will exceed those which are acceptable in connection with the application of printed matter to printed circuit boards and like substrates.
Another requirement which must be met by a stencil for the application of printing ink to a printed circuit board or another substrate which is to be imprinted with the same or with an even higher degree of accuracy is that the filaments of the fabric be stretched by the application of forces which are much larger than those required in connection with the making of many heretofore known stencils. Thus, not only the uniformity of stretching action but also the magnitude of the force which is applied to ensure a uniform stretching action must greatly exceed the degree of uniformity and the magnitude of forces which can be achieved and applied by resorting to heretofore known techniques. The application of larger stretching forces can be achieved only by further enhancing the uniformity of the stretching action because any, even slight, departures from uniform stretching would result in breakage of filaments as soon as the applied force reaches a value which is still below the desired optimum value. In other words, an apparatus which meets the above in part contradictory requirements must be designed to ensure the establishment of a highly uniform stretching action while, at the same time, permitting the application of forces much greater than those which are normally applied in accordance with heretofore known techniques. The presently available stencil making and fabric stretching apparatus cannot satisfy the above requirements.